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Compton Gamma Ray Observatory Second NASA telescope mission after Hubble Launched using the Space Shuttle in April 1991 and operated successfully until it was de-orbited on June 4, 2000 The CGRO carried four instruments for  -ray astronomy, each with its own energy range, detection technique, and scientific goals, covering energies from less than 15 keV to more than 30 GeV 9

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1.The Burst and Transient Source Experiment (BATSE) was the smallest of the CGRO instruments, consisting of eight modules located one on each corner of the spacecraft, consisting of a large flat NaI(Tl) scintillator and a smaller thicker scintillator for spectral measurements, combined to cover an energy range from 15 keV to over 1 MeV. 2.Oriented Scintillation Spectrometer Experiment (OSSE). It used four large, collimated scintillator detectors to study g-rays in the range from 60 keV to 10 MeV. 3. The Compton Telescope (COMPTEL) detected, for medium energy  -rays between 0.8 MeV and 30 MeV, used a Compton scattering technique. 4.The Energetic Gamma Ray Experiment Telescope (EGRET) was the high-energy instrument on CGRO, covering the energy range from 20 MeV to 30 GeV. 10

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19 The LAT is a pair-production telescope. The tracking section has 36 layers of silicon microstrip detectors, with 16 layers of tungsten foil (12 thin layers, 0.03 X 0, at the top or front of the instrument, followed by 4 thick layers, 0.18 X 0, in the back section for γ-ray pair conversion. The tracker is followed by an array of CsI crystals to determine the γ- ray energy and is surrounded by segmented charged-particle detectors (plastic scintillators with PMTs) to reject cosmic-ray backgrounds. The LAT’s improved sensitivity compared to EGRET stems from: a large peak e ﬀ ective area ( ∼ 8000 cm 2, or ∼ 6 × EGRET’s), large ﬁeld of view ( ∼ 2.4 sr, or nearly 5 × EGRET’s), good background rejection, superior angular resolution (68% containment angle ∼ 0.6◦ at 1 GeV for the front section and about a factor of 2 larger for the back section), improved observing e ﬃ ciency

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FERMI-LAT sky map 20

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22 Sky map of the LAT data for the first 3 months, Aito ﬀ projection in Galactic coordinates.  ray intensity for E>300 MeV, in units of photons m −2 s −1 sr −1. The list of sources was obtained after three steps which were applied in sequence: detection, localization, signiﬁcance estimate. Source characteristics (ﬂux in two energy bands, time variability) and possible counterparts

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26 Spectral energy distribution of photons produced in leptonic/hadronic models. Synchrotron radiation is produced by relativistic electrons accelerated in a magnetic field. The produced photons represent also the target for inverse Compton scattering of the parent electrons. When hadrons interact with matter, a distribution of  -rays from  0 decays as indicated by the green curve could be produced. Superimposition of  -rays produced in leptonic and hadronic mechanisms is assumed in case of mixed models

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The 2704 BATSE GRBs 44 Map of the locations of a total of 2704 GRBs recorded with the BATSE on board NASA's CGRO during the nine-year mission. The isotropy of the GRB distribution is evident from this figure. The projection is in galactic coordinates; the plane of the Milky Way Galaxy is along the horizontal line at the middle of the figure

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45 VEDI: GRBs are short-lived bursts of gamma-ray photons. At least some of them are associated with a special type of supernovae; Lasting anywhere from a few milliseconds to several minutes, GRBs shine hundreds of times brighter than a typical supernova, making them briefly the brightest source of cosmic gamma-ray photons in the observable Universe. GRBs are detected roughly once per day, from random directions in the sky by satellite experiments; Until recently, GRBs were the biggest mystery in HE astronomy. They were discovered serendipitously in the late 1960s by U.S. military satellites looking for Soviet nuclear testing in violation of the atmospheric nuclear test ban treaty. These satellites carried  -ray detectors since a nuclear explosion produces  rays.

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46 GRBs are separated into two classes: long- and short-duration bursts. Long duration ones last more than 2 seconds and short-duration ones last less than 2 seconds Long and short duration GRBs are created by fundamentally different physical properties A sampling of the large variety of GRB time profiles, as detected from the CGRO satellite As recently as the early 1990s, astronomers didn't even know if GRBs originated in our Galaxy or at cosmological distances BATSE detector catalogued 2,704 GRBs during nine year lifetime ( ). It was not equipped to make afterglow observations.

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Long and short GRBs 47 Possible candidates for short GRBs are mergers of neutron star binaries or neutron star - black hole binaries, which lose angular momentum and undergo a merger Possible candidates for long GRBs are core collapse of a special kind of very massive star. This core collapse occurs while the outer layers of the star explode in an especially energetic supernova (the “hypernova”, 100 times the SN).

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The Italian BeppoSAX satellite 48 Satellite observations (starting form the Italian satellite Beppo-SAX), follow-up ground-based observations, and theoretical work have allowed astronomers to link GRBs to supernovae in distant galaxies BeppoSAX was equipped with both a  ray and an X- ray detector. It spotted the X-ray afterglow signature associated with the GRB on February 28, 1997 Discovery of the extragalactic origin of GRBs X-ray image of the first BEPPO-SaX GRB

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49 Theoretical models of GRBs Long GRBs: The explosion originates at the center of these massive stars. While a black hole forms from the collapsing core, this explosion sends a blast wave moving through the star at speeds close to the speed of light. The gamma rays are created when the blast wave collides with stellar material still inside the star. Erupting through the star surface, the blast wave of stellar material sweeps through space, colliding with intervening gas and dust, producing additional emission of photons. These emissions are believed responsible for the "afterglow" of progressively less energetic photons, starting with X rays, visible light and radio waves

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The Fireball model 50 The Fireball model is the most widely used theoretical framework to describe the physics of the GRBs. It originates from considerations on the total energy release of a GRB and its extremely short variability time